The present invention relates to a surface defect evaluating apparatus making use of X-ray diffraction topography, wherein X-ray diffraction is utilized to measure the spatial distribution of defects such as lattice defects and lattice distortion contained in a single crystal specimen.
X-ray diffraction topography is capable of observing defects such as dislocation, lamination defects, twinning plane, impurity precipitation and segregation in a single crystal having an excellent quality and a small dislocation density. The resolution of measurement is determined by the size of an X-ray source utilized, the geometric condition of the apparatus, the wavelength width of X-rays, the particle size of a photographic plate, and the like. Various methods have been proposed for X-ray diffraction topography.
According to Lang's method, an X-ray flux is emitted to irradiate a thin sample spot, and the diffracted X-rays passing through the sample are recorded on a photographic plate. By scanning the sample and photographic plate along the crystal plane to be measured, the defect distribution in a wide region of the sample is taken on one photograph.
According to the Berg-Barrett method, the X-ray source is kept away in order to improve the collimation of the X-ray flux through a slit before the X-ray flux enters a sample. Additionally, the photographic plate is placed near the sample surface so that the diffracted X-rays reflected on the surface may enter the photographic plate vertically.
In the double crystal method, the X-ray flux beam is diffracted by a first crystal to improve the collimation before irradiating a second crystal which is the sample to be analyzed, and thereafter the X-rays reflected and diffracted from the sample are recorded on the photographic plate.
In the conventional X-ray diffraction topographic method, however, using a winding-type tungsten filament as the cathode of the X-ray tube, the intensity distribution of electron beams radiated from the cathode is uneven, corresponding to the coarse and dense distribution of filaments. Accordingly, the intensity-distribution of X-rays generated from the anode target is not spatially uniform. Such a non-uniform X-ray flux causes the recording density on the photographic plate to fluctuate spatially, and as a result the measuring precision is greatly impaired.